1. Field of the Invention
The present invention relates generally to protective devices for electrical switchgear and, more particularly, to the protection of electrical switchgear from arcing fault currents.
2. Discussion of the Related Art
Switchgear enclosures are commonly employed in electrical power distribution systems for enclosing circuit interrupters and switching equipment associated with the distribution system. Typically, switchgear enclosures are comprised of a number of individual stacked or adjacent compartments, each of the switchgear compartments receiving electrical power from a power source and distributing the electrical power through a feeder circuit to one or more loads. Generally, each of the switchgear compartments includes circuit interrupters for breaking electric power in a particular feeder circuit in response to hazardous current overloads in the circuit, or normal switching events.
In addition to current overloads, the switchgear enclosure may encounter other hazardous conditions known as arcing faults. Arcing faults occur when electric current “arcs” or flows through ionized gas between conductors, e.g., between two ends of broken or damaged conductors, or between a conductor and ground in the switchgear enclosure. Arcing faults typically result from corroded, worn or aged wiring or insulation, loose connections and electrical stress caused by repeated overloading, lightning strikes, etc. Particularly in medium- to high-voltage power distribution systems, the ionized gas associated with arcing faults may be released at pressures and temperatures sufficient to severely damage or destroy the switchgear equipment and/or cause severe burning injuries or death to operating personnel.
Switchgear enclosures generally provide arc-resistant metal switchgear compartments, often with a means for venting the gases from the compartments in the event of an arcing fault. These compartments are designed to withstand the pressures and temperatures of the gases associated with an arcing fault and reduce the likelihood or extent of damage to switchgear equipment by preventing the gases from entering adjacent switchgear compartments. Safety to operating personnel is enhanced by channeling and venting the hot gases away from operating personnel. However, because these systems do not eliminate the generation and release of hot gases associated with arcing faults, they do not completely eliminate the risk of injury to operating personnel and/or damage to the switchgear equipment.
Therefore, one commonly employed method for enhancing the safety and durability of switchgear enclosures in the event of arcing faults, as described in U.S. Pat. No. 5,933,308 to Garzon, is to provide arc-resistant metal switchgear compartments with a means for grounding or shunting the source bus current in the event of an arcing fault condition. This is done in Garzon by monitoring the rise rate of the source or main bus current and monitoring the light produced by arcing events in each feeder compartment by optical sensors. The current and the optical signals are AND'ed together to produce an arcing fault detection signal which activates an arc diverter mechanism within the appropriate time frame. Other known arcing fault sensing circuits use only optical detectors.
ANSI/IEEE standard C37.20.7 is currently being revised to include low voltage (LV) power switchgear C37.20.1 construction and metal enclosed C37.20.3 construction. The current design of the known arcing fault protectors cannot be used in these low voltage constructions.
In known arc diverter systems using a combination of optical detectors and current sensing, the harmonics on the main line, especially the third harmonic, rise at a rate fast enough to create a positive signal on the current sensor a majority of the time. This leaves the optical arc sensors on the feeder lines as the major determinant. However, low voltage applications commonly have an open air switching and circuit breakers, rather than the vacuum systems of most modern Medium Voltage (MV) switching and breakers. Therefore, in these LV systems detectable light from arcs can be created anytime a switching action or break occurs. Most LV breakers and some older Medium Voltage breakers are open air. Therefore an optical sensor cannot be relied on since the arc diverter would consistently be activated with both current and optical detectors AND'ing together almost every time a switching or breaking event occurs, rather than only when a true arcing fault condition occurs. Therefore, another approach is needed. The present invention is directed to addressing this need.